1. Field of the Invention The present invention relates to a mold used in pressure casting ceramic articles.
2. Description of the Prior Art A non-pressurized slip casting process for ceramic articles has been adopted for long together with a lathe molding process and a dry press molding process Most ceramic articles having especially large size and complex shape have been made by the non-pressurized casting process using a gypsum mold. However, this non-pressurized casting process has a fatal defect obstructing the improvement in productivity. In the non-pressurized casting process, the slip has its water content absorbed into the mold by the mold's capillary action so that the casting rate of the slip on the molding surface cannot be improved drastically. When the gypsum mold is saturated with the water, its capillary action is so weakened that the mold has to be dried up for a long time after every its one or two uses. In order to eliminate those defects concomitant with the non-pressurized casting process, a pressure casting process has recently been developed to propose a variety of pressure molds. However, these molds are accompanied by defects to be urgently solved and raise practical bottlenecks in the pressure casting process.
Specifically, the mold for pressure casting according to the prior art has such a structure that a strong pressure-resisting container or iron box for reinforcement is filled up directly with a slurry or powder (e.g., a mixture of an epoxy resin and sand) for forming a porous layer (as is disclosed in Japanese Patent Laid-Open No. 8010 / 1985 or 208005 / 1983 or U.K. Patent No. 1,295,055, for example).
For this structure, it is remarkably difficult to make the strong pressure-resisting container or reinforcing iron box similar to the cast product or article. Due to this difficulty, the porous layer cannot be made evenly thick so that it is locally very thick. The excessive thickness of the porous layer will increase the compression strain due to the slip pressure at the pressure casting step to make the molding surface of the porous layer liable to be cracked. When the cast product is to be removed from the mold, moreover, there arises another defect that the porous layer is caused to bite the product by the reaction of the compression strain, thus making the removing or demolding step difficult.
In the pressure casting process, furthermore, the water forced at the casting step into the porous layer is drained through passages such as channels. For removing the product from the mold, too, these passages are used to blow compressed air into the porous layer to spurt the water and air from the molding surface of the mold. If the mold is constructed of an upper or top part and a lower or bottom part, for example, the product cannot be removed simultaneously from the upper and lower parts. In the current demolding method, therefore, one mold part is evacuated to attract the product whereas the other mold part is supplied with compressed air to remove the product. Then, the evacuation is released to supply compressed air to that one part thereby to remove the product. Those passages are used to evacuate the porous layer during the demolding step. If the water and air fail to come out evenly from the molding surface at the demolding step, the mold release may be partially degraded to produce defective articles.
Incidentally, the mold of the prior art reinforced by the iron box (as is disclosed in U.K. Patent No. 1,295,055 or U.S. Pat. No. 3,243,860, for example) is so constructed that the iron box is formed with holes through which the water and air are guided to spurt into the porous layer. Since the iron box except for a special one is extremely difficult to be made similar to the product, as has been described hereinbefore, the holes of the iron box are spaced irregularly from the molding surface of the mold, thus raising a defect that the demolding is troubled.
In order to eliminate those defects, there has been proposed a mold which is constructed by fixing a wire net in the inner surface of the pressure-resisting container at a desired spacing from the molding surface of the porous mold, connecting a porous conduit for water and air communications to the wire net with its one end extending to the outside of the mold, and by filling up the inside of the pressure-resisting container with slurry for forming the porous layer (as is disclosed in Japanese Patent Laid-Open No. 208005 / 1983). However, this mold has the aforementioned thick porous layer so that it cannot eliminate the defects of occurrence of the cracks due to the elastic strain at the pressure casting step and the bite of the product by the mold at the demolding step.
In this mold of the prior art, on the other hand, the deformation or breakage of the mold due to the slip pressure in the pressure casting process is prevented by the combined strength of the porous layer and the pressure-resisting container or the iron box. Since the porous layer has a low strength and a small modulus of elasticity, however, the mold is enabled to bear the slip pressure exclusively by the pressure-resisting container or the iron box. Therefore, these container and box have to be drastically strong.
With the structures thus far described, moreover, the mold of the prior art has another defect that the porous layer has to be made thick because the clamping pressure for standing the slip pressure at the casting step is borne by the porous layer having a small modulus of elasticity.